Pentium 4s did 3.8 Ghz 15 years ago :). Granted, they would set fire to your house whilst doing so, but they did it! They also had terrible clock management with barely any OS support for that kind of thing, so they were all constantly at 100% clock speed. (Given the advances in power and clock management, I would guess that most people computers, i.e. desktop, laptops, phones) spend more time at lower clock speeds than they did when people ran Pentium 4s, and still smoke them in performance!)
The problem with upping past 5Ghz is that those Intel chips are really pushing the limits of physics. At 28nm there aren't that many atoms left in a gate, so it's very easy for the doping and stuff to not work correctly and then the gates 'break' very easily. It's only of the reasons clock speed has stagnated. Also, as the chips get more transitions on them things like clock skew become even worse.
Because of these physical limits faults are more common during manufacture, so these days the CPU binning is much broader, i.e. i7s are cherry picked i5s, whereas it used to be you made a bunch of Pentiums and cherry picked the crap ones to be the lower clocked ones. It's also one of the reasons they moved of tick/tock and onto the new 3-stage stepping to eek out more time on each manufacturing set up.
So whilst IPC has been going up massively due to multi-core/threads, architectural improvements, power management and smaller transistor sizes, CPU clock speeds have really stagnated at around 4Ghz.
Performance, however, has continued to increase :) And that's what's important.
(Though, it's worth pointing out, that due to this stagnation of clock speeds, single-thread performance, which you harp on about in that qualcomm thread, has barely changed over the years. Single thread performance definitely isn't following the classic Moore's law trend (though that's about transitions))
ps: I watched an interesting video the other day that's relevant to this thread: